75 ohm adapters & fixed vs. variable RCA's

Just bought a new tuner Rotel-2100. I'd like to use the 75ohm vs. the 300ohm for my antenna. I have the 75ohm to 300ohm splitter but I am under the belief the 75ohm works better? However, it is the old fashioned 75ohm, not with just the thin pin in the middle. Now for the problem and question. I even have the connector to go from the modern 75ohm to the old 75 ohm but the recess in the unit is to narrow for the outside of it? Anybody know where to get this part? My second question is, when I hook this tuner up do I use the fixed or variable rca's? Which one is preferable? Thanks.